Coatings on substrates may serve various purposes, ranging from decorative purposes through to protection from harmful effects on the coated substrate. In order to be able to exert each of these functions, it is vital that the coating adheres to the coated substrate surface sufficiently well, such that there is no unwanted delamination between the coating and the coated surface.
Coatings are notably in layer form or film form depending on their thickness, with in each case only one surface of the coating being joined to the surface of the substrate to be coated. This of course is not to rule out the possibility of this arrangement of the substrate with coating being repeated ad infinitum, which may lead, for example, to a substrate/coating/substrate laminate or substrate/coating/substrate/coating laminate. A further characteristic of coatings is that they generally have a largely homogeneous film thickness, of the kind usual for surface-coating films or coats. Another characteristic of such coatings is that the material to be coated is not mixed or intermingled in any way with the coating material, and so the layer-form or film-form coating is in contact only by one of its sheetlike surfaces with the surface of the coated material. Sheetlike surfaces are to be understood hereinafter not only as level surfaces, but also as uneven surfaces, such as, for example, bent, domed, waved, creased or otherwise nonuniformly designed surfaces. Thus, for example, the surface of a wire is also a sheetlike surface of a body in the sense of the invention.
On account of the physical and chemical properties, coatings are often produced using plastics, in order to be able to provide laminates comprising the coated substrate and the coating having the desired properties. One possible example of a composite of this kind is a metal body with sheetlike extent whose surface is coated with a polyurethane. Where the coating has a relatively low film thickness, it is referred to preferably as a surface-coating film, which is based customarily on a physically and/or chemically cured surface-coating system as coating material. The coating material may optionally also be present in a modified form, such as in filled or foamed form, for example.
Vital to all forms of coatings, i.e., to layer-form coating and to film-form surface-coatings, is the existence, between the coated surface and the adjoining surface of the coating or film, of a sufficient adhesion, which is lasting and which ensures the desired purpose, as for example a decoration and/or a protection against different, preferably harmful effects.
An overview of various classes of substance used for promoting adhesion is given in Progress in Organic Coatings 1995, 26, 275. From this it is evident that, generally, adhesion-promoting substances are used in different ways: such as (i) either as a pretreatment layer, in which case the adhesion-promoting substance is first deposited on the substrate surface and only then is the intended coating applied; or (ii) as an additive directly in the intended coating, with the advantageous consequence of a reduction by one workstep relative to method (i).
According to the prior art, one class of substance which is widely used and is added to coatings for the purpose of promoting the adhesion is the class of the low molecular mass, organofunctional alkoxysilanes.
The use of organosilanes as direct pretreatment on the metal surface to be coated is described in, for example, J. Oil Colour Chem. Assoc. 1982, 65, page 415, and the use of organosilanes as additive in the coating material is described in the same publication on page 436. Furthermore, for example, the pretreatment of copper or aluminum with silanes is described in Electrochimica Acta 2006, 51, 6097.
J. Adhesion Sci. Technol. 2006, 20, 1615 describes improved adhesion of an epoxy varnish on an aluminum substrate surface through use of a combination of glycidyloxypropyltrimethoxysilane and a further hydrophobic silane.
Prog. Organic Coatings 2006, 57, 307 describes the influence of vinyl- and aminosilanes on the adhesion between epoxy clearcoats and aluminum surfaces.
According to Proceedings: 28th Annual Meeting of the Adhesion Society, Feb. 13-16, 2005, page 173 ff. and also 486 ff., the use of epoxy-functional organosilanes improves adhesion of epoxy varnishes on aluminum substrate surfaces and of epoxy varnishes on glass substrates, respectively.
EP 1157146 describes the pretreatment of a metal surface with specific bis-silyl-silanes, enhancing the adhesion of rubber to this substrate.
WO 2008/003190 describes the use of addition products of thioalkoxysilanes with polyfunctional (meth)acrylates as adhesion promoters for radiation-curable systems. Since these known addition products still have free terminal (meth)acrylate groups, they may be incorporated as well by radical crosslinking in the course of a radiation cure, and this may lead to unwanted secondary reactions. Moreover, on storage, for example, the polymerization tendency of the double bonds in these products may cause them to undergo partial or complete polymerization, thereby possibly leading to loss of or adverse effect on their functionality when they are employed. A further factor is that adhesion promoters with double bonds as functional groups are limited to employment in radically curing (radiation-curing) binder/polymer systems. Adhesion promoters with universal applicability, i.e., adhesion promoters for different binder/polymer systems, however, are a requirement of the users.
This requirement is also not met by the reaction products of hydroxy-functional (meth)acrylates with isocyanate-functional alkoxysilanes, described in U.S. Pat. No. 4,889,768, which are employed in conjunction with glass fibers, since the addition products likewise still have terminal (meth)acrylate groups.
Furthermore, WO 2008/003191 describes the use of (1) addition products of isocyanatosilanes with OH- or NH-functional (meth)acrylates, or of (2) addition products of diisocyanates with OH- or NH-functional (meth)acrylates, and the subsequent reaction thereof with thio- or aminosilanes, with these adhesion promoters as well still containing free terminal (meth)acrylate groups which may be incorporated by crosslinking as well, radically, in the course of the radiation cure when they are employed for radiation-curable systems.
WO 2009/064282 describes addition products of aminosilanes with preferably low molecular mass (meth)acrylates as adhesion promoters for polyurea coatings on substrates; their adhesion does not meet all of the requirements.
It was an object of the present invention, therefore, to avoid the disadvantages of the prior art and to achieve excellent adhesion between coatings, preferably surface-coating films, based on any of a very wide variety of polymers, and substrate surfaces composed of a multiplicity of materials.